1. Field of the Invention
The present invention relates to a process for reducing plant availability of heavy metals and the use of cross-linked poly(meth)acrylates in said process.
2. Background of the Invention
Until now, no method is known by which it has been possible to eliminate the effect of heavy metals on plants in soils contaminated with heavy metals.
At present, two methods are used for planting crops in fields containing heavy metals: either top soil is added to the contaminated soils, or the contaminated soil is stripped away and replaced by top soil, in which case the new top soil layer must usually be more than one meter in thickness. All these methods require much effort and are very expensive.
Experiments conducted so far to reduce plant availability of heavy metals by adding polyacrylates have either been unsuccessful (O. A. El-Hady, A. A. Lotfy, B. M. A. El-Hady, Egyptian Journal of Soil Science, 30:4, 545-557, 1990) or had the effect that the availability of heavy metals such as lead, nickel, manganese or iron was clearly increased (R. L. Mikkelsen, Fertiliser Research 41, 87-92 [1995]; J. J. Mortvedt, R. L. Mikkelsen, J. H. Kelsoc, Soil Science Society of America Journal, 56:4, 1319-1324 [1992]; F. Awal, L. Kluge, R. Kluge, J. Abadia in Iron Nutrition in Soils and Plants, Proceedings of the Seventh International Symposium, Zaragoza, Spain, Jun. 27 to Jul. 2, 1993, Kluwer Academic Publishers: Dordrecht, 1995, 53-62; J. J. Mortvedth, R. L. Mikkelsen, A. D. Behel Jr., Journal of Plant Nutrition, 15:10, 1913-1926 [1992].
EP 415 141 discloses soil conditioners consisting of super-absorbers containing sulfonic acid which have a high water retention ability even after the repeated absorption/desorption of saline solutions. As a result, heavy-metal ions are released into the plants and are not retained for long periods.
EP 72 213 A describes the use of super-absorbers with a hydration rate of 16-80% in mixtures of free-flow agents for the improvement of water retention or aeration. This document provides no information about working cross-linked polyacrylates into the soil for the long-term absorption of heavy metals.
Surprisingly it was found that when cross-linked poly(meth)acrylates are added to substrates contaminated with heavy metals, the plant availability of heavy metals is distinctly reduced or even completely eliminated.
Heavy metals contained in the substrates are captured by the negatively charged carboxyl groups of the poly(meth)acrylates, which means that they are no longer available to the plants. This effect remains active over a long period of time, surprisingly as long as many months. In particular, this was unexpected by persons skilled in the art, and it allows a long-term elimination of the effect which heavy metals have in contaminated soils.
The substrates, such as soils or hydrocultures, are treated with the cross-linked poly(meth)acrylates, in particular by mixing in quantities of 0.1% to 2.5% in relation to the total substrate weight, preferably quantities of 0.5% to 2.0%, in particular about 0.2% to 0.8%, and specifically 0.4% to 0.6%.
The soil is excavated to a depth of up to about 50 cm, preferably 30 cm, and in particular about 40 cm, and mixed with the poly(meth)acrylates.